Kruskal–Szekeres coordinates

Summary

Kruskal–Szekeres coordinates are coordinates on the maximally extended Schwarzschild spacetime. They cover the whole spacetime with one chart, and radial light rays are straight lines at $45$ degrees, as in flat spacetime. The extension they reveal has four regions: two exterior regions, a black hole interior and a white hole interior. They are the standard picture of the global structure of the Schwarzschild metric.